WASHINGTON (CBSNewYork/AP) — A New York-born gunman opened fire inside a building at the Washington Navy Yard on Monday morning, killing 12 people and wounding three others, including a law enforcement officer, authorities said.

The gunman, Aaron Alexis, 34, of Fort Worth, Texas, was killed in a shootout with police at the Naval Sea Systems Command building, authorities said.

Alexis was born in Queens, according to the FBI. He was employed part-time doing clerical work at Borough of Manhattan Community College from 2001 until 2003, the school said.

Aaron Alexis' Brother-In-Law: He Wasn't Close With Family

Alexis’ mother, sister and brother-in-law live in Bedford-Stuyvesant, Brooklyn, where FBI agents were interviewing the family and searching their Putnam Avenue home Monday.

“When I got here, (the family was) very distraught, very stressed out, tears,” Anthony Little, Alexis’ brother-in-law, told CBS 2’s Jessica Schneider. “They didn’t see it coming. It’s very hurtful. And their hearts are going out more to the victims.”

Little — who is married to Alexis’ sister, Naomi, 32 — admitted that he never met Alexis, adding that the man had not spoken to some members of his family in more than five years. Family members, however, never expressed concern that Alexis was dangerous, Little said.

Michael Ritrovato, who said he was a close friend of the gunman’s, told Schneider that Alexis was upset recently about getting stiffed on a job.

“What he called me about three to four months ago (was) that they didn’t pay him on a new job as a government contractor or something,” Ritrovato said. “He didn’t get paid. They took him to Tokyo and there was no pay, and that’s what he stressed to me.”

Wendy Lopez, a former neighbor, said she was questioned by the FBI, too.

“Asked me the same questions everyone else is asking — what I remembered about the family, anything in his demeanor to indicate anything,” she said. “But there was nothing at all.”

Aaron Alexis (credit: handout)

Authorities had been looking into the possibility that other gunmen might have been involved in the shooting, but they ruled out that possibility Monday night.

“We do now feel comfortable that we have the single and sole person responsible for the loss of life inside of the base today,” District of Columbia Police Chief Cathy Lanier said.

The FBI said Alexis gained entry to the yard by using a valid pass he obtained from doing contract work there. The FBI is seeking more information about Alexis by asking anyone who knew him to call 800-CALL-FBI or log onto https://tips.fbi.gov.

“While we have learned some information about his recent whereabouts, we continue to work to determine where he has been, who he has talked to and what he has done,” said Valerie Parlave, assistant director of the FBI’s Washington office. “This includes determining the origins of the weapons he used.”

A law enforcement official told CBS News’ Pat Milton that Alexis apparently had a handgun, an AR-15 assault rifle and a shotgun with him and was wearing dark blue clothing.

Officials said in addition to the 13 casualties, eight other people were injured — three of whom were shot and five others who suffered either injuries from falling or due to stress. None of the injuries is considered to be life-threatening.

The ages of the 12 victims killed by Alexis ranged from 46 to 73 years old, authorities said. Police released a partial list of victims’ names Monday night:

Michael Arnold, 59

Sylvia Frasier, 53

Kathy Gaarde, 62

John Roger Johnson, 73

Frank Kohler, 50

Kenneth Bernard Proctor, 46

Vishnu Pandit, 61

‘HE AIMED HIS GUN AT US AND THEN FIRED’

Witnesses described a gunman opening fire from the fourth floor, aiming down on people in the first-floor cafeteria. Others said a gunman fired at them in a third-floor hallway.

As CBS 2’s Dick Brennan reported, Navy Commander Tim Jirus was helping evacuate people when a victim was gunned down right in front of him.

Others described the chaos and tense moments as the gunfire broke out.

“As we were exiting the back door, we noticed him down the hall,” Navy Yard worker Todd Brundidge said of the shooter. “And as he came around the corner, he aimed his gun at us and then fired at least two or three shots. And we ran down the stairs to get out of the building and as we left the building, there were still shots in the building.”

“I would be lying to you if I told you we weren’t scared, but I was very pleased with my folks that we were not panicked,” U.S. Navy Capt. Mark Vandroff said.

About 3,000 people work at the Naval Sea Systems Command headquarters, many of them civilians.

Patricia Ward, a logistics management specialist, said she was in the cafeteria and heard shots. They sounded like “pop, pop, pop,” she said. After a few seconds, there were more shots, she said.

“Everybody just panicked at first,” she said. “It was just people running, running, running.”

Police and federal agents from multiple law enforcement agencies responded to the scene. Ambulances were parked outside, streets in the area were closed, and departures from Reagan National Airport were temporarily halted for security reasons. Helicopters evacuated victims from rooftops, and they were taken to nearby hospitals.

Both Lanier and D.C. Mayor Vincent Gray said there is no reason to think the attack was a terrorist attack, but noted that nothing has been ruled out.

Aaron Alexis mug shot (credit: Fort Worth Police)

REACTION FROM ELECTED OFFICIALS

Mayor Michael Bloomberg on Monday afternoon said this fatal shooting incident is another grim reminder that there needs to be reforms to gun control in America.

“There are just too many guns around, and it just happens again and again and again,” he said. “People are getting killed. Innocent people are getting killed. I don’t know how long we can go and continue to do this. We’re the only developed country in the world that has a gun problem like this.”

Though the incident in D.C. appeared to be isolated, Bloomberg said the NYPD beefed up its presence at some potential targets in New York City.

“Whenever there is an event like that, we always ramp up our security,” said Bloomberg.

Speaking Monday afternoon, President Barack Obama said the victims were “courageous Americans” and “patriots” who knew about the risks of serving overseas, but wouldn’t have expected such “unimaginable violence” at home.

Aaron Alexis (credit: handout)

GUNMAN’S CHECKERED PAST

A profile began to emerge Monday evening of Alexis, who was a former Navy reservist, a Defense Department contractor and a convert to Buddhism. He also had flashes of temper that led to run-ins with police over shootings in Fort Worth, Texas, and Seattle.

While some neighbors and acquaintances described him as “nice,” his father once told detectives in Seattle that his son had anger management problems related to post-traumatic stress brought on by the terror attacks of Sept. 11, 2001.

Alexis lived in Seattle in 2004 and 2005, according to public documents. In 2004, Seattle police said, Alexis was arrested in 2004 for shooting out the tires of another man’s vehicle in what he later described to detectives as an anger-fueled “blackout.” Police later arrested Alexis, searched his home, found a gun and ammunition in his room, and booked him into the King County Jail for malicious mischief.

Alexis told police he could not remember firing his gun at the Honda until an hour after the incident.

He served in the Navy reserves from 2007-11. It was while he was still in the reserves that a neighbor in Fort Worth reported she had been nearly struck by a bullet shot from his downstairs apartment.

In September 2010, Fort Worth police questioned Alexis about the neighbor’s report; he admitted to firing his weapon but said he was cleaning his gun when it accidentally discharged. The neighbor told police she was scared of Alexis and felt he fired intentionally because he had complained about her making too much noise.

Alexis was arrested on suspicion of discharging a firearm within city limits, but Tarrant County district attorney’s spokeswoman Melody McDonald Lanier said the case was not pursued after it was determined the gun discharged accidentally.

Alexis was taking online classes through Embry-Riddle Aeronautical University in Fort Worth, school officials confirmed.